This application claims priority from European Application for Patent No. 02291025.1 filed on Apr. 23, 2002, the disclosure of which is hereby incorporated by reference.
1. Technical Field of the Invention
The present invention relates to the generation of digital signals coding amplitude values of different analog signals having a set frequency.
2. Description of Related Art
Sensor electrodes, detecting physical features or events, produce a periodic wave tension value (an analog signal) whose amplitude is a function of the physical features or events detected by the sensor. It is useful for further processing to digitize the amplitude values of the analog signal. It is known in the prior art to use analog to digital converters (ADCs) for generating each digital signal that codes the amplitude values of a detected analog signal.
Some state of the art devices, with an ADC able to sample many points in a period of the analog signal, determine the amplitude of the analog signal by the maximum of a curve comprising many digital points, with each digital point coding an analog point on the analog signal. A device with many sensors is cumbersome because there are as many ADCs as sensors. Furthermore, high-speed ADCs consume a lot of power during operation. This poses a problem for miniaturized electronic devices, such as organizers or mobile phones.
One solution is to valuate the amplitude of the analog signal before digital conversion, for example, with a filter or otherwise. This allows the use of lower speed ADCs converting only one value per period. Thus, the individual ADC is less cumbersome and consumes less power. However, this does not satisfy the needs of miniaturization, because with a large number of ADCs the power consumption is still high and a device is still cumbersome. Also, another problem arises when many ADCs are used because of the necessary calibration to lessen the resolution loss that occurs when all of the ADCs in a device are not identical.
Another solution is to reduce the number of ADCs by successively connecting a group of sensors to the same ADC via an analog multiplexer. However, it is difficult to substantially reduce the number of ADCs because the same sensor can be connected again to the ADC only after a number of periods of the analog signal equal to the number or, at best, to half the number of the sensors of the group. Each valuation of the amplitude needs a total period or, at best, a half period of the analog signal received by an ADC. The smaller the quantity of ADCs, the greater the time separating two samples of the same analog signal. For example, connecting each sensor during a full period to a unique ADC requires again connecting the same sensor after a number of periods equal to the total number of sensors. This delay between two digital conversions for the same analog signal is not satisfactory when amplitude values vary at a high rate. If a small number of sensors are connected to the same ADC, this still will require a large number of ADCs for all the sensors. Thus, the resulting structure is still cumbersome and still consumes a lot of power during operation.
One embodiment according to the principles of the present invention is a particularly compact device having a number (P) of first single analog inputs, each receiving a first analog signal having a frequency (F) from a sensor electrode, and a first single digital output for sequentially generating P first digital signals coding the amplitude values of the first analog signals.
An embodiment according to the principles of the present invention comprises:
1) a first multiplexer having a number (M) of first multiple analog inputs with each input comprising a number (N) of second single analog inputs, each receiving an analog signal, with the numbers N and M chosen so that their product is at least equal to P, a first multiple analog output having N second single analog outputs, and a first command input connecting the first multiple analog output sequentially to one of the first multiple analog inputs;
2) a phase sensitive amplifier having N third single analog inputs, each connected to a separate second single analog output of the first multiplexer, N third single analog outputs, and a second command input for generating on each third single analog output an analog amplitude value of the analog signal received on corresponding third analog input;
3) a second multiplexer having N fourth single analog inputs, each tied to a separate third single analog output of the phase sensitive amplifier, a fourth single analog output, and a third command input for connecting the fourth single analog output sequentially to one of the fourth single analog inputs;
4) an ADC having a fifth single analog input tied to the fourth single analog output of the second multiplexer, a second single digital output tied to a first output of the device, and a fourth command input for generating a digital signal coding the amplitude of the analog value received on the fifth single analog input; and
5) a front-end controller operable for setting the first command input with a first new value, the second command input with a first pulse value at the beginning of each period of the first analog signal, the third command input with a succession of N second new values, and the fourth command input with a succession of N second pulse values at a time delayed after the beginning of each period of the first analog signal.
The present invention allows considerable reduction of the number of ADCs, and therefore, the size of the device. Preferably, N is equal to the number of analog amplitude values for which the ADC is suited to generate digital signals coding each analog amplitude value during a period of the first analog signal. Further, each first new value is set for connecting the first multiple analog output on a different first multiple analog input during a period of the first analog signal so as to connect the first multiple analog output on all the first multiple analog inputs during M periods of the first analog signal. Also, each second new value is set for connecting the fourth single analog output on a different fourth single analog input so as to connect the fourth single analog output on all the fourth single analog inputs during a period of the first analog signal. In this embodiment, one ADC is sufficient to scan all the analog signals received by the first single analog inputs.
Particularly, the device comprises a wave generator with at least one fifth analog output for yielding a second analog signal having a frequency and an amplitude that is programmable by the front-end controller. The resulting analog signal is a periodic signal such as a sine wave or any other periodic signal suited for synchronization.
More particularly, for detecting or measuring, the present invention comprises at least one stimulation electrode tied to the fifth output and P sensor electrodes connected to each of the second single analog inputs.
For special applications such as commands activated by touching a finger on a surface, the stimulation electrode and the sensor electrodes are printed on a flexible band so as to constitute a finger print captor. The first and second multiplexers, the phase sensitive amplifier, the ADC, the front-end controller and the wave generator are encapsulated in an integrated circuit connected to the flexible band.
Another embodiment is a method according to the principles of the present invention for generating a number (P) of first digital signals each coding an amplitude value of one first analog signal having a set frequency (F) comprising the steps of:
1) sampling a number (N) of first analog signals and measuring the amplitude value for each of the N first analog signals sampled during a period of the first analog signal;
2) successively generating N digital signals, each coding the analog amplitude value of the N first analog signals during the same period of the first analog signal; and
3) iterating M times the first and second steps for different first analog signals so as to sample a complete number (P) of first analog signals.
The first step allows sampling and measuring a complete subset of analog signals at once during a same period of an analog signal. The second step allows sequentially digitizing the subset""s measurements during the same period. The third step allows digitizing a complete set of analog signal measurements with a short delay between two subsequent digitalizations of a same subset, increasing the real time performance of the digital measurements.
Advantageously, to measure physical features of a body not generating analog signals, the method comprises a fourth step for continuously yielding a second analog signal having a periodic form with programmable frequency and amplitude, and for applying the second analog signal to the body so that a set of P different points on the body each generate one of the first analog signals with an amplitude value the result of the combination of the programmable amplitude and the physical features of the corresponding point on the body.
Particularly, the first digital signals are computed to detect movement of the body when the set of P different points change corresponding to a changing physical feature.
Also, the first digital signals are computed to draw a map of points of the body along its changing physical features generating the first analog signals.